1. Field of the Invention
The present invention relates generally to heat pump systems and, more particularly, to a motor-compressor protection technique for such a system.
2. Description of the Prior Art
In heat pump systems designed to provide both cooling and heating for an enclosed space such as a room or entire building, a typical installation would have only the inside condenser/evaporator coils within the building and the outside condenser/evaporator coils and motor-compressor combination mounted outside of the building. Mounting such equipment outside of the building not only provides more usable space within the building but also removes the relatively noisy equipment from inside the building. However, placing the motor-compressor combination on the outside of the building subjects it to the weather which, in some locals, produces a wide range of ambient temperatures.
For motor-compressors used in such systems, there are practical limitations placed upon the thermal parameters of the motor and the work load to which it can be subjected before it will be damaged. Conventionally, limiting sensors were placed on the motor and the system turned off if the limits were exceeded. In addition, for the heat pump system itself, limit switches are conventionally placed on the high pressure, condenser side of the compressor and on the low pressure, evaporator side of the compressor so that if the pressure in the condensing side were exceeded, possibly indicating a blockage of the system, the motor-compressor would be turned off. Similarly, if the pressure in the evaporator side dropped below a fixed limit, possibly indicating leakage of the refrigerant, the motor-compressor and system would also be turned off. While temperature limit switches and the pressure limit cutoff switches protect the motor-compressor to a certain extent, conventional prior art practice has these sensors operating on fixed values and, since certain of the limiting factors and the damage point for the motor-compressor are not fixed values but are variable with various associated conditions, in some cases the motor-compressor could be operated under conditions where it would be damaged without any of the protecting sensors being activated.
Thus, conventional prior art practice in motor-compressor protection techniques for heat pump systems have not been completely satisfactory in that substantially complete protection is not attained and, under certain conditions, the motor could be damaged despite the proper operation of such prior protection techniques.